1. Field of the Invention
This invention relates to a relative azimuth measuring method and apparatus for measuring an azimuth angle and an elevation angle, that is, a relative azimuth, of a target with respect to a target tracer using an optical signal, and more particularly to a relative azimuth measuring method and apparatus which can measure a relative azimuth to a target at a short distance with a high degree of accuracy.
2. Description of the Related Art
Conventionally, one of relative azimuth measuring apparatus of the type mentioned is known as a rendezvous laser radar which is used for orientation of the azimuth of a target spacecraft which is a target in rendezvous docking of artificial satellites. The relative azimuth measuring apparatus measures the azimuth angle and the elevation angle of the target spacecraft with respect to a chaser spacecraft as a target tracer, or in other words, the relative azimuth between moving bodies or the relative azimuth of a moving body as viewed from a fixed point.
In the following, a relative azimuth measuring apparatus which employs conventional techniques is described with reference to a block diagram of the conventional relative azimuth measuring apparatus shown in FIG. 1 and a front elevational view of reflector 102 which is shown in FIG. 2 and employed in the relative azimuth measuring apparatus of FIG. 1.
The present relative azimuth measuring apparatus includes measuring section 101 installed fixedly on a chaser spacecraft or a like chaser, and reflector 102 installed fixedly on target 1 such as a target spacecraft. Measuring section 101 measures the relative azimuth of target 1 to measuring section 101. Here, reflector 102 is an aggregate of a plurality of small reflectors 1021 formed from corner cubes or like elements and reflects light to its incident direction. All small reflectors 1021 have an equal reflection factor and size and a same profile.
Light source 5 in measuring section 101 is a laser light oscillator or a like element which generates light of the wavelength of 830 nm, and generates emergent light H1 of the known wavelength. This emergent light H1 is emitted toward reflector 102 by way of half mirror 4 which makes a part of optical system 3. Reflector 102 receives emergent light H1 and reflects it toward the direction of half mirror 4. Reflected light H2 from reflector 102 is introduced again to and passes through half mirror 4 so that it is introduced to two-dimensional imaging element 6 for which a CCD or a like element is employed.
Imaging element 6 forms an image of reflected light H2 as a two-dimensional reflector image, photo-electrically converts the reflector image into electric signals S1 of corresponding two-dimensional image information and supplies electric signals S1 to angle calculating circuit 108. It is to be noted that the reflector image indicates an outer profile of reflector 102. Angle calculating circuit 108 calculates the center of the reflector image from electric signals S1 and calculates the displacement between the center of the reflector image on the imaging plane and the center of the field of view of imaging element 6, and then calculates the azimuth angle and the elevation angle, that is, the relative azimuth, of target 1, on which reflector 102 is installed fixedly, with respect to measuring section 101.
However, the conventional relative azimuth measuring apparatus is disadvantageous in that, since the size of the reflector is fixed, when the target and the measuring section approach each other to decrease the distance between them, the reflector image increases in size so that it protrudes from the field of view of the imaging element, resulting in difficulty to accurately find out the center of the reflector image and in degradation of the accuracy in measurement of the relative azimuth of the target with respect to the measuring section.
The relative azimuth measuring apparatus is also disadvantageous in that it cannot measure the relative azimuth any more when the entire field of view of the imaging element is covered with the reflector image.